This type of torque measuring device is disclosed in Japanese Patent No. 169326. In this device, a shaft having soft magnetic and magnetostrictive properties is formed on its outer periphery with a pair of knurled regions inclined in opposite directions to form angles of 45 degrees with respect to the axis of rotation of the shaft, each knurled region having an exciting coil and a detecting coil wound along the outer periphery thereof.
According to such arrangement, the knurled regions provide magnetic anisotropy, and a change in permeability occurring in each knurled region according to transmission torque is detected by the detecting coil. At this time, when a compressive force acts on one of the knurled regions in the knurling direction, a tensile force acts on the other knurled region since the knurled regions are inclined in opposite directions. As a result, the detection voltage on one of the detecting coils increases with increasing torque while the detection voltage on the other detecting coil decreases. If the detection voltages on these detecting coils are differentiated, a torque detection voltage indicating a change in torque can be obtained.
Another conventional torque detecting device is disclosed in Japanese Patent Application Laid-Open Specification No. 59-166827. In this device, instead of forming the knurled regions shown in aforesaid Japanese Patent No. 169326, amorphous soft magnetic layers inclined in opposite directions are formed on the surface of a rotary shaft by bonding, plating or the like. According to such arrangement, measurement of torque is possible also concerning a shaft having no magnetostrictive property.
With these conventional arrangements, however, the balance of detection voltages outputted from the two detecting coils with no transmission torque being imposed tends to change owing to temperature changes and time-dependent changes. This is due to changes in magnetic properties (permeability and magnetostriction factor) of the material of the shaft and changes in iron loss caused by magnetic lines passing through the shaft surface; and variations in the quality of exciting coils, detecting coils and magnetic layers on the shaft according to material and manufacture. As a result, the sensitivity, the zero point (output value when the torque is zero) and the like changes, causing error in measured torque values.